The way we have described speech sounds so far was in terms of
how they are produced by manipulating the vocal apparatus and
how they can be transcribed with phonetic symbols on paper.
We can also describe sounds in terms of how we can hear them. How we can hear a sound depends on
its acoustic structure.
Acoustics: the science of the physical properties of sounds.
Acoustic Phonetics: the science of the physical properties of speech sounds.
Propagation of sound
A line of people waiting to buy tickets to a concert is useful analogy for a sound wave.
The movement of the person at the front of the line creates a gap (‘rarefaction’); this gap travels
through the line.
Then the first person is shoved back into the second person, creating more crowdedness
(‘compression’); the crowdedness also travels through the line.
• Every time we say something we create speech sound waves
• The air pressure either:
i. rises: the air particles/molecules get close to each other – compression, or
ii. falls: the air particles get further away from each other – rarefaction
1 Sound Waves and Waveform
Sound wave a traveling pressure fluctuation that propagates through a medium.
Pressure fluctuations impinging on the eardrum produce the sensation of sound.
Pressure fluctuations are often cyclic, repetitive.
The waveform: based on the pressure variation. It represents the same shape as that of the movement
of an air molecule.
Waveform is represented with the time dimension on the horizontal axis and the amplitude dimension
on the vertical axis.
Simple vs. complex sound waves
Simple Periodic Waves
Pure tones, when represented on a waveform, will appear as sine waves – simple periodic waves.
Sine waves are smooth and symmetrical s-shaped waves.
Amplitude and Intensity
• Amplitude and intensity are directly related to each other and are physical properties of sound waves
that correlate with the psychological property of loudness.
• The greater the amplitude or intensity of a sound wave, the louder the sound.
Amplitude: degree of variation in air pressure from neutral to higher and lower (measured in hPa
[hecto-Pascal] or mb [millibar])
Intensity: power transmitted by the wave (measured in dB [decibels])
• Frequency is a physical property of sound waves that correlates with the psychological property of
• The greater the frequency of a sound wave, the higher the pitch of the sound.
• Frequency is measured in terms of number of cycles per second (measured in Hertz (Hz)).
Some Intensities: •60 dB normal conversation
•120 dB threshold of pain •30 dB whispered conversation
•110 dB rock concert
•0 dB threshold of audibility
Some Frequencies •225 Hz average woman’s speech
•20,000 Hz highest perceptible
•120 Hz average man’s speech
•265 Hz average child’s speech
3 •20 Hz lowest perceptible
4 Complex Repetitive Waves
• Speech is more complex than a pure tone and is not realized as a sine wave but rather as a complex
repetitive (periodic) wave.
• The figure on the right represents a complex repetitive wave, which is a combination of the two waves
on the left 3 (100 Hz and 1000 Hz).
• Fourier analysis is a mathematical technique capable of analyzing all complex repetitive waves as the
sum of a series of simple sine waves.
• It is believed that a human ear performs a Fourier-like analysis.
Periodic, Aperiodic and Transient
1. Periodic Sound Waves
Eg. vowels, approximants, nasals - sonorants
5 2. Aperiodic Sound Waves
no repetitive cycles, but random noise
Eg. fricatives, aspiration
3. Transient Sound Waves
last very shortly
Eg. stop burst
Individual component waves of a complex wave
•The harmonic with the lowest frequency is called fundamental, and its frequency is referred to as the
fundamental frequency or F0.
•The frequency of the complex wave as a whole is the same as the frequency of the fundamental.
•All harmonics above the fundamental have a frequency that is a whole multiple of the fundamental.
• A spectrum is a display that shows the amplitude (or intensity) of each harmonic.
• The figure on the right presents a spectrum of the complex repetitive wave from the figure on the left.
•The natural tendency of a body to vibrate at certain frequencies is known as resonance.
•By changing the shape of our vocal tract (through movement of the tongue and lips) we change the
resonating properties of the vocal tract.
•The shape of the vocal tract in the production of any particular sound will reinforce some frequencies
and filter out others.
•The flow of air through the glottis as the vocal folds are opening and closing produces a wave, called
glottal wave, which will appear on a spectrum as a series of harmonics occurring at regular intervals
(with each harmonic being a multiple of a fundamental).
•The harmonics gradually decrease in intensity as they become higher in frequency.
8 •When a glottal wave passes through the vocal tract, some groups of harmonics will be reinforced and
others will be reduced.
•The clusters of harmonics that are reinforced by the resonating properties of the vocal tract are called
•For neutral vowel or uniform tube, following frequencies are intensified: ∼500Hz (F1), ∼1330Hz (F2),
∼2200Hz (F3), ∼3200Hz (F4)
•A spectrogram is a display that is particularly useful when examining speech sounds.
•It has three dimensions:
1.) Time: plotted on the x-axis (horizontal)
2.) Frequency: plotted on the y-axis (vertical)
3.) Intensity: darkness of representation (greater intensity = darker)
•In this course, we will be working mostly with spectrograms.
9 •There are two kinds of spectrogram displays.
Narrow-band spectrograms make it easy to distinguish individual harmonics.
Wide-band spectrograms make it easy to see formants, as well individual cycles of waves
(due to fine temporal resolution).
10 Spectrograms and Harmonics
Spectrogram and Formants
aperiodic waves are also called noise
• Up till now, we have been examining periodic sounds sounds that can be characterized in terms of a
repeating waveform such as produced by the vocal folds in voiced sounds.
• There are, however, waves that are aperiodic, involving random fluctuations of amplitude (or
• Aperiodic waves are also called noise.
11 •The kind of sound produced in a voiceless fricative is also aperiodic:
rather than being generated by repetitive vibrations, it results from random turbulence in the
airflow caused by an abrupt narrowing of the vocal tract.
12 Sound Source
the air expelled from the lungs; in voiced sounds, vocal cords produce complex sound waves by
changing the air pressure
these sound waves are periodic
can be broken down; the components are called harmonics
•first harmonic - fundamental frequency f0
•harmonics higher than f0 - second, third, fourth harmonic, etc.;
multiples of f0
•complex sound wave reaches the upper vocal cavity
•the shape of the or